The sailing schedule tells you when a vessel is supposed to leave. It does not tell you how often that line actually keeps that date on this lane.
Schedule reliability varies by carrier and by trade, and it is public information most shippers never look at before booking on price alone. On a lane with a tight production window, a cheaper booking with a lower on-time percentage can cost more in expedited road transport at the other end than it saved in freight.
When we recommend a service, the reliability figure for that specific string is part of the quote, not a footnote you have to ask for.